FIG. 1 illustrates how telephone exchanges are placed in a telephone network. Reference numerals 10 and 12 identify two local exchanges to which subscribers 20 are connected. Reference numeral 11 identifies a transit exchange which is capable of connecting traffic between different local exchanges. The telephone exchanges 10, 11, 12 in the network illustrated in FIG. 1 are connected to one another by means of trunk connections 30. The telephone exchange 11 in the specific network shown in FIG. 1 is a tandem exchange, because direct trunk lines are found between all local exchanges. The feature of interest in the FIG. 1 network is that traffic can be moved between the local exchanges 10 and 12 through the tandem exchange 11. The reference numeral 40 identifies signalling connections for so-called common channel signalling. A typical feature of present-day telephone networks is that the signalling connections are separate from the trunk connections. In the case of old techniques, signalling is effected on the trunk connections, for instance with tones on the trunk connection between the exchanges. As mentioned, the signalling network has been separated from the trunk connections in latter-day techniques. Signalling in the telephone network takes place between the network telephone exchanges and is intended to control the traffic, including among other things the transfer of cradle-switched subscriber states and of address information relating to called subscribers.
FIG. 2 illustrates generally one of the telephone exchanges 10, 11 or 12 of the telephone network illustrated in FIG. 1. More specifically, only those functions which are relevant to technical problems that are fundamental to the present invention are described. The telephone exchange, in this case the local exchange 10, comprises a computer 100 which controls coupling equipment. The coupling equipment comprises a subscriber interface 101, switching equipment 102, and a trunk connection interface 103. The switching equipment 102 functions to connect telephone channels for telephone calls between the subscriber interface 101 and the trunk connection interface 103. A computer 100 controls signalling from the subscriber 20 to the local exchange 10, via a subscriber line 25 and the subscriber interface 101. Similarly, on the trunk side, the computer 100 controls connection of trunk circuits 30 to the switching equipment 102 via the trunk connection interface 103. The computer 100 also has an interface 104 towards the signal network, via a signal terminal. The computer performs a number of items of software 110 which have different functions in the telephone exchange, such as to control the connection of calls or maintenance functions, for instance the collection of billing information. These items of software 110 can signal with items of software in other telephone exchanges, via the signal terminal 104. When the software 110 signals with software in other telephone exchanges, the protocols referenced P40 in FIG. 2 are used. These protocols are thus used in the signal connections 40, although they are shown separately in FIG. 2.
The signalling technique that predominates in telephone networks today is based on common channel signalling in accordance with CCITT or ANSI recommendations.
When a telephone connection is to be established from one telephone exchange to another, i.e., when a subscriber has lifted the receiver of a telephone set and has sent to the telephone exchange 10, via the subscriber interface 101, the number of subscriber B to which the call shall be established and the digits of the B-number have been identified in the telephone exchange 10, it is assumed in the illustrated case that the subscriber to which the number dialled relates is not present in the own telephone exchange 10, but in another telephone exchange, for instance the telephone exchange 12 in FIG. 1. In order to establish a call connection to the called subscriber, the B-subscriber, in the local exchange 12, there is used a so-called circuit related signalling process, in which a trunk connection 30 is selected between the telephone exchanges 10 and 12 and in which signalling refers to a selected circuit in the trunk line. Typically, there is chosen in the trunk line a time slot that is able to transmit a telephone call. The signals are transmitted with reference to this time slot and signalling takes place with a protocol P40 that can be interpreted by respective software in the two telephone exchanges. Thus, the connection is not effected directly from software to software in the telephone exchange 10 and 12, and instead the telephone exchange 10 signals to the telephone exchange 12 by referring to the chosen physical circuit that is used. The protocol used when signalling is, for instance, TUP (Telephony User Part) or ISUP (Integrated Services Digital Network User Part). Thus, it is this protocol P40 that is used by the software 110 over the signalling connection 40 in order to establish the telephone call connection on the trunk line between the telephone exchanges.
In the case of this signalling procedure, solely telephone exchanges that have direct trunk connections therebetween are able to communicate with one another. If, in accordance with the FIG. 3 illustration, a telephone call is to be connected between two subscribers 21 and 22 which are connected to different local exchanges 10 and 12 and which lack direct trunk connections 30 between one another, it is necessary to establish the call connection through the agency of one or more transit telephone exchanges 11. This is because the signals must always have a reference to a circuit in the trunk connections.
FIG. 3 illustrates simply a transit case between three telephone exchanges, i.e. between two local telephone exchanges 10 and 12 and a transit exchange 11, which passes traffic between the local exchanges 10 and 12. In the illustrated case, the A-subscriber 21 in the local exchange 10 has requested a call to the B-subscriber 22 in the other local exchange 10. Thus, when establishing the call connection, no signalling takes place directly between the local exchanges 10 and 12, and signalling must therefore pass via the transit exchange 11. Actually, the call connection between the local exchanges 10 and 11 is divided into two connection halves which are mutually connected in the transit exchange. The local exchange 10 selects a trunk circuit to the transit exchange 11 and uses a protocol P41 to establish one half of the connection to the transit exchange 11. The transit exchange 11 establishes a connection half to the local exchange 12, by selecting a trunk circuit on the trunk line 30 and using a protocol P42 to establish the other connection path to the local exchange 12. When the local exchange 12 has accepted the call, the transit exchange 11 connects the call from the A-subscriber 21 and is, in principle, transparent to the call.
The telephone service offered to subscribers 21 and 22 is dependent on the software in all telephone exchanges 10, 11, 12 active in a connection and also on those protocols P41 and P42 that are used by the exchanges for communication therebetween. If a new function is to be introduced, for instance automatic callback in the event of an engaged number, it is necessary for this function to be implemented in the software in all telephone exchanges that are active in the process of establishing a connection from subscriber A to subscriber B, and also that the protocol P41 and the protocol P42 used between the software are capable of handling and controlling the new function. Thus, it is necessary to modify all three telephone exchanges and all protocols when introducing a new function. Since telephone calls can be connected through many different paths through a telephone network in one country, it is necessary for all telephone exchanges to support the functionality that is offered to subscribers in the local exchanges. This is a disadvantage, particularly when desiring to provide in the network functions that are directed towards a specific subscriber category, for instance the subscribers in a particular company. Even though the company may have subscriber connections in only a few telephone exchanges, for instance four working places in a town are connected to different telephone exchanges in the same town, it is not sufficient to modify the software in these telephone exchanges, since a call may pass along different routes through the network of the country concerned. It is therefore necessary for the protocol between different telephone exchanges to be mutually compatible when wishing to introduce the new functionality. When the company concerned is spread over many different working places in the country, it is necessary to update a very large number of different telephone exchanges.
Another problem encountered with present-day telephone networks concerns the routing of telephone calls from one subscriber to another. Typically, telephone numbers and telephone networks are built-up hierarchically, so that different parts of a telephone number relate to different geographical regions and areas. Normally, outlying regions are preceded with an area routing number which corresponds to a routing number area serving as a routing number exchange which handles incoming traffic to all subscribers located within this routing number area. Further parts of the telephone number, normally the first two or three digits of the number, are used to identify the local exchange to which given subscribers are connected. Referring to FIG. 4, there is shown a number of local telephone exchanges 10, 12, 14, 15, of which exchanges 10 and 14 are connected to a routing number exchange 16 located in a routing number area 18, whereas the local telephone exchanges 10 and 15 are connected to another routing number telephone exchange 17 which is located in another routing number area 19. The routing number telephone exchanges 16 and 17 are connected to a transit exchange 11. Assume that a telephone call shall be connected from one subscriber belonging to exchange 10, furthest to the left in FIG. 4, to a subscriber connected to the local exchange 12, furthest to the right in FIG. 4. The local exchange 10 will not recognize the B-number of the subscriber in the local exchange 12, but will transfer the B-number to its routing number exchange 16, which in turn recognizes that the number is concerned with another routing number area and therefore transfers the call to a transit exchange 11. The transit exchange 11 passes the call to the routing number area of the B-subscriber, more specifically to the routing number exchanges 17 in the routing number area 19. The routing number exchange 17 analyzes further parts of the B-number, in order to establish the local exchange to which the B-subscriber is connected, and switches the call to this local exchange, in the illustrated case the local exchange 12. The disadvantage with this type of telephone call routing procedure is that a subscriber is forced to change telephone numbers when he moves between the areas served by different telephone exchanges. A subscriber may also be forced to change his telephone number when it is necessary to move his connection from one telephone exchange to another in conjunction with restructuring or extending the telephone network. This represents a disadvantage to the individual subscriber and may also involve additional costs to the network manager, who must plan restructuring of the network in good time and inform all subscribers concerned with this restructuring of the changes that will be made to the subscriber numbers. Information of this nature incurs heavy costs.
Modern telephone networks include a further possibility of signalling directly between telephone exchanges and other equipment in the telephone network. Such signalling is based on the ability of the software in a telephone exchange or in some other equipment to address functions directly in another telephone exchange or in some other equipment. This enables purely signal channels to be established between different items of equipment in the telephone network, without relating the signalling process to a trunk circuit when connecting a call. This direct addressing procedure is used typically to address central functions in the telephone network, such as so-called free-phone services (In Sweden 020-numbers, in the U.S.A. 800-numbers) or to call subscriber data bases in the mobile telephone network.
FIG. 5 shows a typical example which includes the same units as those shown in FIG. 3 with the addition of central equipment 13 which permits conversation to be made with the aid of a protocol P43 which is tailored to the functionality provided by equipment 13. This protocol P43 is separate from the protocols P41 and P42. When a subscriber 21 requires access to the service concerned, for instance an 020-number with regard to intelligent networks, or an 010-number with regard to mobile telephone networks, the telephone exchange 10 discovers that it is unable to handle the number requested and therefore transfers the call to the telephone exchange 11, which discovers that the call is directed to a directory number which requires particular treatment, and therefore calls the software in the telephone exchange 3 which can handle this directory number. The equipment 13, typically a network data base, responds to the query from the telephone exchange 11 by disclosing the telephone number to which the call from subscriber 21 shall be routed. Signalling between the telephone exchanges 11 and 13 is not circuit-related, and when the telephone exchange 11 has learned of the destination to which the call shall be routed, signalling is effected to the relevant telephone exchange with the aid of circuit-switched signalling in the aforedescribed manner. The use of free signalling when calling software in the equipment 13 in order to obtain an answer to where a requested functionality is found in the network can be employed to solve the problem with the fixed relationship of a telephone number with the geographical location of a subscriber when the equipment 13 is comprised of a network data base which keeps an account of where the subscribers are located. The drawback with such a solution, which thus means that free signalling can be used and central functions that are capable of performing execute special services are obtained in the network, is that it is necessary to modify the software in selected, standard telephone exchanges, the telephone exchanges 10, 11 and 12 in the FIG. 5 illustration. For instance, it is necessary for the telephone exchange 11 in FIG. 5 to include software which will recognize a mobile telephone number 010 or an 020-number. Exchanges which do not recognize these numbers shall transfer the number upwards in the exchange hierarchy. The exchange which recognizes the numbers must be capable of starting communication with the data base and use a separate protocol for this purpose. From the aspect of establishing connections, the ability of only some telephone exchanges to identify such service-calling numbers (010, 020, and so on) is a disadvantage. For instance, in the case of a mobile telephony network about 80% of the traffic is comprised of traffic that passes between an exchange and a mobile subscriber, both of which are located in the near region of a telephone exchange. If the telephone exchange 10 were to have the functionality of being able to recognize such a service-calling number, it would also be possible to introduce an optimal routing selection in the network for the establishment of a connection to the B-side. It is costly to modify software in telephone exchanges. Consequently, a trade-off is made between those gains that can be made from a call connection aspect by modifying the software in each telephone exchange and the costs entailed by such software modification. However, there is a need for a telecommunication network in which many different types of services can be introduced and identified in a similar way, in addition to those services that are identified by such 010 and 020 numbers. It would be disastrous if it was necessary to modify software in each of the telephone exchanges within the network with a protocol corresponding to the protocol P43 which is adapted to the specific service to be introduced, when introducing a new service into the network.